At the points of steam-turbine casings where the shaft of the machine passes through the casing, devices which prevent the ingress of air into the low-pressure turbine stages and also prevent the escape of steam into the atmosphere from casing sections of higher pressure must be provided. In this case, the only suitable seals are essentially non-contact seals, which of course exhibit residual leakage quantities.
Systems in which a barrier sealing system prevents the escape of steam from the high-pressure shaft seal are therefore normally implemented. The leakage steam is thus drawn off into a separate system instead of flowing into the atmosphere. This steam may expediently be directed to the low-pressure shaft seal, where it flows out as sealing steam and displaces air from the shaft seal.
The high-pressure leakage quantity and the sealing-steam quantity for the low-pressure shaft seal are ideally in equilibrium; however, systems via which excess leakage steam is drawn off, for example, into the condenser or, conversely, supplemental sealing steam is supplied are normally provided. In this case, the supplemental-sealing-steam feed, in particular in transient operating states, is of importance if the pressure in the high-pressure casings, for example during start-up of the machine, is not yet sufficient in order to deliver a sufficiently large amount of sealing steam. In the event of highly throttled or closed control valves, sealing steam must even be supplied to the high-pressure shaft seals.
The supplemental sealing steam is normally fed from the live-steam line. Thus sealing steam having the high thermodynamic data of the live steam is available at the turbine inlet, and this sealing steam is successively reduced, for example by water injection, to states which are adapted in particular to the material temperature of the shaft and casing of the steam turbine at the respective sealing point.
In transient operation of a turbine, the feeding of live steam into the sealing-steam system, in particular during start-up or an emergency trip, results in inadmissibly large jumps in temperature, which affect the casing and the shaft journals. Such sudden changes in the live-steam data can be corrected by water injection. However, the reduction in the steam temperature by means of water injection is not without its problems especially at high-pressure shaft seals. There is the risk of unevaporated water encountering the hot shaft, a factor which in turn leads to undesirable thermal shocks.
The problems which occur during start-up of a plant can in the meantime be coped with by a sufficiently slow increase in the temperature at the outlet of the superheater of the boiler--that is to say the boiler firing temperature. In particular during the operation of combined-cycle plants, this means that the gas turbines have to be operated at a very low load over a longer period. Apart from the poor efficiency of such an operation, such an operating mode of the plant results in operational difficulties which are not to be underestimated.
At this point, EP 0 605 156 B1 proposes to provide means of feeding the live-steam line from different temperature levels of the boiler. During the start-up of the turbine, the feed of the steam superheated to the maximum extent to the live-steam line is completely or partly interrupted. In this case, the live steam is supplied from an intermediate stage of the superheater at reduced temperature level. Admixing of saturated steam from the boiler drum provides a further means of controlling the live-steam temperature.
Apart from the very high equipment cost, which is necessary to realize the method disclosed by EP 605 156 B1, the thermodynamic state of the steam, which is directed as additional sealing steam to the sealing-steam system, is still linked to the live-steam state. The result of this is that, even when the circuit specified in this publication is implemented, measures for conditioning the sealing steam in certain operating states are necessary. In particular during an emergency trip of a steam turbine, even when the circuit and the method according to EP 0 605 156 B1 are used, there is therefore a high risk of subjecting the shaft and the casing to inadmissibly large jumps in temperature, in particular in the region of the high-pressure shaft seal. This is because, during operation of the machine, the steam from the high-pressure shaft seal expands at least over some labyrinth tips and is therefore already considerably cooler than the live steam. However, during an emergency trip of the turbine, markedly hotter live steam is suddenly fed to the high-pressure shaft seal. As mentioned above, even water injection cannot compensate for this jump in temperature and also leads to the risk of applying water droplets to the surface of the hot shaft.